This invention relates generally to signal receivers and more particularly to receivers having adaptive interfering signal cancellation.
As is known in the art, considerable effort has been made in recent years to provide adaptive interfering signal cancellation. One such interfering signal is a radar system jammer signal. In such application, there are two components from the jammer which must be canceled: a direct path component; and an indirect path component. The indirect path component results from jammer signals which bounce from adjacent clutter (i.e., terrain bounces) which extend over a wide azimuth angle and are received by a mainbeam of the radar system receiver antenna even though the jammer is positioned in a sidelobe of the antenna. These multi-path terrain bounce signals may adversely effect the effective azimuth target detection coverage of the radar system.
One technique used to suppress interfering signals is described in U.S. Pat. No. 4,720,712 entitled xe2x80x9cAdaptive Beam Forming Apparatusxe2x80x9d issued Jan. 19, 1988, inventors Brookner et al, assigned to the same assignee as the present invention, the entire subject matter contained therein being incorporated herein by reference. Such U.S. Patent refers to the use of Sample Matrix Inversion techniques to provide main lobe nulling of the antenna beam to suppress an interfering signal. The Sample Matrix Inversion (SMI) technique is described in an article entitledxe2x80x9cRapid Convergence Rate in Adaptive Arraysxe2x80x9d, by I. S. Reed, J. D. Mallet, and L. E. Brennan, published in the xe2x80x9cIEEE Transactions on Aerospace and Electronic Systemsxe2x80x9d, volume AES-10, No. 6, November, 1984. pages 853-863. Still other beamforming and adaptive cancellation techniques are described in an article entitledxe2x80x9cBeamforming: A Versatile Approach to Spatial Filteringxe2x80x9d by Barry D. VanVeen and Kevin M. Buckley, published in IEEE ASSP Magazine, April 1989, pages 4-24.
One technique used to suppress direct and indirect paths of an interfering signal includes the use of an adaptive processor having time delay and Doppler frequency taps. Adaptive processors using these taps achieve almost ideal performance at the expense of processing throughput. An important feature in the adaptive processor is the correlation between the jammer signal in the direct path and the jammer signal in the indirect path. When the direct path and the indirect path signals received by the antenna system are correlated, cancellation of sidelobe direct path and mainbeam (i.e., mainlobe) indirect path occurs without requiring mainlobe nulling as described in the above-reference U.S. Patent. In general, however, the direct path and the indirect path signals are somewhat de-correlated. The source of this de-correlation is due to differential Doppler frequency and time delay induced phase shifts. Differential Doppler frequency results when there is a relative velocity between the radar system receiver antenna and the jammer. The amount of de-correlation is dependent on the geometric relationship between the jammer, terrain (i.e., clutter) illuminated by the jammer and the radar system receiver antenna.
In accordance with one feature of the invention a receiver is provided having an array of elements adapted to receive an interfering signal from both a direct path and from an indirect path. The direct and indirect paths have both differential Doppler frequency and differential time delay induced phase shifts. A plurality of receiver sections is provided. Each one of the receiver sections is fed by a corresponding one of the elements. An adaptive interfering signal canceler is fed by the plurality of receiver sections for providing an adaptive beam forming network to suppress both the direct and indirect paths of the interfering signal. The adaptive canceler separates the signal received by the elements into a plurality of sub-intervals, determines frequency components over a pre-determined bandwidth in each of the sub-intervals, forms a plurality of frequency sub-bands from the determined frequency components, provides adaptive beam forming networks for each of a corresponding one of the frequency sub-bands in each one of the sub-intervals to suppress the differential time delay phase shifts in the received interfering signal, and combines outputs from the plurality of frequency sub-band beam forming networks over the plurality of sub-intervals to produce a composite output signal having suppressed the differential Doppler frequency induced phase shifts in the received interfering signal.
In accordance with another feature of the invention, a radar system receiver is provided having an antenna system adapted to receive a jammer signal from both a direct path and from an indirect path, such direct and indirect paths having both differential Doppler frequency and differential time delay induced phase shifts. The antenna system includes a plurality of antenna elements. A plurality of radar receiver sections, each one having a clutter filter, is provided. Each one of the receiver sections is fed by a corresponding one of the antenna elements. An adaptive jammer canceler is fed by the plurality of radar receiver sections for providing a beam to suppress both the direct and indirect paths of the jamming signal. The adaptive canceler includes a frequency transformation section for determining frequency components in each of a plurality of frequency bands in signals received from the jammer by each one of the plurality of antenna elements over a predetermined frequency bandwidth. The adaptive canceler also includes a covariance matrix fed by the frequency sub-bands of the determined frequency components for each of the plurality of antenna elements for determining covariance matrix coefficients for each of the frequency sub-bands. A beam forming network is fed by the frequency sub-bands and the covariance matrix coefficients for forming an antenna beam for each of one of series of sub-intervals of a radar dwell to suppress the differential time delay phase shifts in the received jamming signal. Inverse frequency transformation and combiner sections are provided for forming a composite output signal from the antenna beams produced over the series sub-intervals to suppress the differential Doppler frequency induced phase shifts in the received jamming signal. The composite output signal represents the beam provided by the adaptive canceler to suppress both the direct and indirect paths of the jamming signal.
In accordance with still another feature of the invention, a method is provided for canceling both differential Doppler frequency and differential time delay induced phase shifts produced by direct and indirect paths of an interfering signal received by an array of elements adapted to receive the interfering signal. The method includes the step of determining, for each a plurality of radar dwell sub-intervals, frequency components of signals received by each of the elements in each of a plurality of frequency sub-bands over a predetermined frequency bandwidth. For each of the plurality of sub-intervals, a beam is produced for each one of a determined frequency sub-bands in each sub-interval. The beam is adaptively formed to suppress the differential time delay induced phase shifts in the received interfering signal during the sub-interval. The beams produced for each one of the sub-intervals in a dwell are combined into a composite beam for the dwell, the composite beam being adapted to suppress the differential Doppler frequency induced phase shifts in the received interfering signal.
In accordance with still another feature of the invention, a method is provided for canceling both differential Doppler frequency and differential time delay induced phase shifts produced by direct and indirect paths of an interfering signal received by an antenna system. The antenna system includes a plurality of antenna elements adapted to receive the interfering signal. The method includes the step of determining for each a plurality of sub-intervals: (a) frequency components of signals received by each of the antenna elements in each of a plurality of frequency sub-bands over a predetermined frequency bandwidth; and, (b) covariance matrix coefficients for each one of a plurality of frequency sub-bands of the predetermined bandwidth in response to the signals in such one of the frequency sub-bands received by the plurality of antenna elements. A beam is produced for each of the plurality of sub-intervals for each one of the frequency sub-band from the covariance matrix coefficients determined in each sub-interval. The beams are adapted to suppress the differential time delay induced phase shifts in the received interfering signal during the sub-interval. The beams produced for each one of the sub-intervals in a dwell are combined into a composite beam, the composite beam being adapted to suppress the differential Doppler frequency induced phase shifts in the received interfering signal.
With such systems and methods, an adaptive canceler is provided which utilizes a balance between both narrow nulling bands and short sub-intervals. More particularly, over a relatively large radar dwell, differential Doppler frequency induced phase shift of the received jammer signal will change over the dwell (i.e., for a constant Doppler frequency, the phase will change linearly over the dwell). Further, over a large frequency band, differential time delay induced phase shift of the received jammer signal will change over the dwell. In accordance with the invention, instead of generating covariance matrix coefficients over the entire dwell (which results in such coefficients nulling an average differential Doppler frequency shift over the entire dwell), each radar dwell is divided into a plurality of sub-intervals to generate, for each relatively short sub-interval, covariance matrix coefficients thereby reducing the differential Doppler frequency induced phase shifts. Also, by dividing the entire frequency bandwidth being processed into frequency sub-bands and processing each frequency sub-band to produce the covariance matrix coefficients for each frequency sub-band (i.e., for each sub-interval a plurality of frequency sub-band beam forming networks are configured, one beam forming network for a corresponding one of the frequency sub-bands), differential time delay induced phase shifts are reduced.
Further, with the invention, significant reduction is made in computation by making a balance between differential Doppler frequency performance and differential time delay performance in suppressing direct and indirect signal paths of a jammer or interfering signal. More particularly, signal processing parameters important to the correlation of the jammer""s direct and indirect path signals are the nulling bandwidth (i.e., nulling sample time) and the nulling time interval (i.e., nulling frequency resolution). The direct and indirect signals are completely de-correlated when the differential time delay is equal to the reciprocal of the nulling bandwidth or when the differential Doppler frequency is the reciprocal of the nulling time interval. More particularly, controlling the adaptive nulling bandwidth and time interval are used as an adaptive cancellation technique. The nulling time interval may be made short, (i.e., divided into sub-intervals) to drive the system immune to differential Doppler frequency induced phase shifts. Alternatively, the nulling bandwidth may be reduced (i.e., use sub-banding) to drive the system immune to differential time delay induced phase shifts. Ideally, both short time intervals and narrow nulling bands could be used to drive the system to be immune to both differential Doppler and time delay. However, the two approaches are in direct conflict. As the nulling interval is shortened (in either frequency or time) the number of data samples of the received signal available for covariance matrix coefficient generation is reduced. As the ratio of the number of samples to the number of degrees of freedom in the adaptation decreases, the covariance matrix coefficient loss increases which results in decreased system performance. To provide sufficient data samples when using short sub-intervals, the system is driven to larger nulling bandwidths which results in degraded differential time delay performance. On the other hand, using narrow nulling bands necessitates the use of increased sub-interval time to provide sufficient samples which thereby results in degraded differential Doppler performance. Thus, a processor that utilizes narrow nulling bands and short sub-intervals to mitigate the impact of the indirect path jamming is a balance between differential Doppler frequency performance, differential time delay performance and covariance matrix loss. There are applications with few elements or low level indirect jamming where this balance is achievable and provides a low cost, effective solution to jammer multipath.